1. Field of the Invention
The invention relates to activated carbon adsorbents. More particularly, it relates to enhancing the performance thereof for advantageous use in pressure swing adsorption processes.
2. Description of the Prior Art
Activated carbon and other adsorbent materials capable of selectively adsorbing a component of a gas mixture can advantageously be employed in pressure swing adsorption (PSA) processing used to separate and purify hydrogen or other desired gases present in available feed gas streams. The PSA process is well known in the art, as evidenced by such patents as Wagner, U.S. Pat. No. 3,430,418 and Fuderer et al, U.S. Pat. No. 3,986,849. Hydrogen in a feed gas, for example, can conveniently be separated and purified from other components of a feed gas stream, i.e., carbon dioxide, nitrogen, carbon monoxide, methane, ethane and the like, by means of the PSA process.
In the PSA, the feed gas stream is typically passed to an adsorbent bed, or to a PSA system comprising a number of such beds, at an elevated adsorption pressure, with a purified product effluent gas being recovered from the adsorbent bed at the elevated adsorption pressure level. The selectively adsorbed components(s) of the feed gas are thereafter desorbed from the adsorbent material and removed from the bed at a lower desorption pressure level. As cyclic processing operations continue, the adsorbent bed is repressurized to the elevated adsorption pressure, and depressurized to the lower desorption pressure, on a continuing basis. In various multi-bed embodiments of the PSA process, such cyclic operations occur in overlapping processing sequence in a number of adsorbent beds as will readily be appreciated by those skilled in the art.
The heat effects that occur at the adsorption and desorption pressures, and at intermediate pressure levels, are found to be of significance in the overall performance of the PSA process. As the process is essentially adiabatic, the heat of adsorption causes an increase in bed temperature during each adsorption step. Because of this increased temperature, the final loading of the adsorbed components is lower than would be the case if no such increase in temperature were to occur. As the adsorbent bed is depressurized to its lower desorption pressure and during the typical purge step to facilitate desorption and bed regeneration, on the other hand, the adiabatic desorption action results in a decrease in bed temperature. Because of this decreased temperature during desorption, the residual adsorbate loading is higher than would be the case if no such decrease in temperature were to occur during desorption. The fluctuation in bed temperature thus reduces the differential loading of the bed in two ways, i.e., by causing lower than desired loading at the end of adsorption and higher than desired residual loading at the end of desorption. This undesirable difference between the highest and lowest temperatures in a pressure swing adsorption cycle can, in practical cases, exceed 30.degree. C., and it can reduce the difference in adsorbate loading of the bed between the end of the adsorption step and the end of the desorption step, i.e., the delta loading, by as much as 50% in some cases. It will be appreciate that the development of convenient means for reducing bed temperature fluctuations during PSA operations would be advantageous in the art. One approach to achieving such a reduction relates to the use of improved adsorbent materials capable of being used in the PSA processing cycle with lower bed temperature fluctuations than occur using the presently available adsorbents.
It is an object of the invention, therefore, to provide an improved adsorption suitable for use in pressure swing adsorption operations.
It is another object of the invention to provide a process for modifying conventional activated carbon adsorbent so as to improve its operating performance in the practice of the PSA process.
It is a further object of the invention to provide a process for modifying conventional activated carbon adsorbent material such as to reduce the temperature fluctuations of such material when employed in the PSA process for gas separation.